An example of a clamping mechanism employing retaining screws for clamping a cutting head to a tool holder adaptor is described in U.S. Pat. No. 4,575,292 and is shown in FIG. 1. When this type of clamping mechanism is used in cutting tools having relatively small diameters a certain problem arises that is negligible or does not arise at all, for large diameter cutting tools. This problem entails the distortion of portions of the cutting tool body as a result of radial forces that exist between the retaining screws and the cutting tool body and the fact that for small diameters the body of the tool will distort under the influence of these radial forces.
FIG. 1 will be used as a reference to illustrate the distortion problem, which will be described with reference to FIGS. 2 to 5 that show various sectional views of the tool shown in FIG. 1. However, not all the features of the cutting tool shown in FIG. 1 are relevant to understanding the problem at hand. Therefore, FIGS. 2 to 5 are only illustrative sectional views of FIG. 1 showing only relevant features. Furthermore, for illustrative purposes some of the spaces between various parts and the distortions thereof have been exaggerated. FIG. 1 shows a cutting tool assembly comprising a cutting head 2 clamped to an adaptor 1. The cutting head has a cutting insert 30 mounted at its front end, and is provided with a shank 5 at its rear end. The adaptor has a bore 3 at its front end in which the shank 5 is co-axially mounted.
The periphery of the adaptor is provided with a threaded through bore 8a (This numeral has been added. The rest of the numerals in FIGS. 1 to 5 are as in the '292 patent.). The shank 5 is provided with a conical bore 7 capable of receiving therein a retaining screw 8 which is screwingly engaged into the threaded bore 8a. When mounting the cutting head on the adaptor, the shank 5 of the cutting head is inserted into the bore 3 of the adaptor until the cutting head abutting surface 6 abuts against the adaptor abutting surface 4. The retaining screw 8 is now screwed into the threaded bore 8a in order to clamp the cutting head 2 to the adaptor 1. The clamping of the cutting head can be considered as a two step process. In the first step the retaining screw is fully screwed into the threaded bore, enters the conical bore 7, but is not firmly tightened. The screw's conical turned end 9 abuts against the conical bore 7 thereby pushing the shank 5 away from the threaded bore 8a towards the adaptor inner wall remote from the threaded bore and at the same time pushing the shank rearwardly relative to the adaptor 1. At this stage the cutting head cannot be released from the adaptor, but it is not securely clamped in position. In the second step the retaining screw is firmly tightened (see FIG. 2), ensuring that the axial component Fa of the retaining force F forces the cutting head 2 rearwardly relative to the adaptor 1 whereby the firm abutting of the cutting head abutting surface 6 against the adaptor abutting surface 4 is obtained.
Such a clamping mechanism has a disadvantage that limits the use of the cutting tool assembly in some milling operations due to instability and inaccuracy problems, as will be described below.
FIG. 3 is a cross-sectional view of a cutting tool assembly similar to that shown in FIG. 2 taken in a plane perpendicular to the axis 10 of the adaptor 11 in the region of contact between the conical end 9 of the screw and the conical bore 7.
As can be seen in FIG. 3, prior to forcibly screwing the retaining screw 18, there is a gap between the shank 25 and the adaptor 11. This gap, of dimension h1, is due to the difference between the outer diameter d1 of the cutting head shank and the inner diameter d2 of the adaptor. This gap which, for illustrational purposes, is shown to be uniform all around the adaptor bore is the required clearance which is essential for the free insertion of the cutting head shank into the adaptor bore. The required clearance is typically in the order of magnitude of 0.02 mm.
FIGS. 4 and 5 show the result of fastening the retaining screw by means of the action of the radial component Fr of the retaining force F, as a two step process. In the first step, as shown in FIG. 4, the retaining screw 18 radially pushes the cutting head shank 25 away from the screw through bore 18a towards the adaptor inner wall 26 remote from the bore 18a . When the shank 25 touches the wall 26, the distance between the through bore 18a and the shank 25 becomes h2, which is twice the initial distance h1 between the shank 25 and the adaptor bore 23. In the second step, as shown in FIG. 5, in order to ensure firm abutting of the cutting head shank 25 against the inner wall 26 and, also, to ensure self-locking of the retaining screw 18, the screw must be forcibly additionally screwed. The greater the radial component Fr of the retaining force F the greater the reaction force Rf acting on the screw in the opposite direction to the force Fr. FIG. 5 shows the assembly of FIG. 4 after the retaining screw 18 was forcibly additionally screwed. As can be seen, the forced screwing distorts the adaptor 11, in the screw's axial direction, to a distance h3 between the cutting head shank 25 and the through bore 18a. The distance h3 is greater than the distance h2. This means, in practice, that the shank is not best supported in the adaptor bore, and the shank 25 and the adaptor 11 are not co-axial, which, of course, leads to inaccuracy and instability of the cutting tool assembly.
The disadvantages of the clamping mechanism described above are more pronounced in small diameter adaptors where the thickness of the adaptor wall t1 is relatively small. Clearly, the thinner the adaptor wall the greater the distortion for a given tightening force of the retaining screw. In order to overcome this disadvantage the wall thickness can be increased, but, this may give rise to other difficulties when performing deep pocketing applications because the adaptor external diameter may become larger than the cutting head diameter, a fact that limits the use of the cutting head to depths less than its length.
Another example of a clamping mechanism employing retaining screws for clamping a cutting head to a tool holder adaptor is described in U.S. Pat. No. 4,511,294. FIG. 6 is a cross-sectional view similar to FIG. 5 in '294. The reference numerals used herein are compatible with the reference numerals used in '294 with 100 added to them.
As shown in FIG. 6, a stub 110 of a shank 103 is inserted into the legs 108 of an adaptor 104. The stub having radially extending threaded bores 113. The legs having radially extending holes 111. The axes of the threaded bores 113 and the holes 111 are parallel and offset relative to one another. When screws 119 having conical heads are inserted into the holes 111 and fully screwed into the threaded bores 113 they urge the rear edge face 114 of the legs 108 against the respective surface 115 of the shank's shoulder 116. As can be seen, firm abutting is obtained by the axial relative movement between the shank 103 and the adaptor 104 whereby the rear end portions of the legs 108 are wedged into the spaces which are inwardly adjacent to the respective surfaces 115 of the shank's shoulder 116 thus preventing the legs 108 from moving away from each other. In accordance with '294 firm abutting between the adaptor 104 and the shank 103 is not obtained solely by the radial tightening of the screws 119 but also requires the wedging of the rear end portions of the legs 108 against the respective shank's shoulder 116. The latter being obtained by means of the axial displacement induced by the tightening of the screws 119.
Consideration will now be given to the effect of the radial tightening of the screws only. This is done by considering an assembly similar to that shown in FIG. 6 but without the wedging of the rear end faces 114 of the legs 108. Also, the conical head of each screw 119 is replaced by a cylindrical head. Such an assembly is shown in FIG. 7.
Again, it is mentioned that the spaces between the various parts and the distortions thereof, as illustrated in FIGS. 8 and 9, have been exaggerated for clarification purposes.
FIG. 8 shows the relative disposition between the shank 103 and the adaptor 104 prior to fastening the screws 119. A gap 120 of dimension a1 is the clearance between the shank 103 and the adaptor 104. The gap 120 is the necessary clearance for the free insertion of the shank 103 into the adaptor 104. In this position, the legs 108 of the adaptor 104 are peripherally equidistanced from the shank 103.
When the screws 119 are firmly tightened, as shown in FIG. 9, the legs 108 are pressed against the shank 103 by a radial retaining force Fra. In this position, the gap 120, along a line connecting the axes of the screws 119, diminishes. However, contact between the shank 103 and adaptor 104 takes place just in the vicinity of the screws 119 due to the construction of the legs 118. This introduces a clamping disadvantage since the adaptor 104 could move relative to the shank 103 when it is subjected to a radial force Frt transversely directed to the radial retaining force Fra.
It is the object of the present invention to provide a cutting tool assembly in which the above mentioned disadvantages are greatly reduced, or overcome.